Conventional trains typically rely on a pneumatic brake system to provide reliable braking. Although individual brake systems may vary slightly, they typically include a main reservoir, located on a locomotive, that supplies pressurized air through a brake pipe to each rail car connected in series to the locomotive. The brake pipe supplies the pressurized air to a combined auxiliary and emergency reservoir on each rail car. A control valve on each rail car senses pressure in the brake pipe to control actuation of the brake system on each rail car and re-charging of the combined reservoir.
For service braking, an operator slowly vents brake pipe pressure. For example, the operator may vent brake pipe pressure 6-8 pounds per square inch for minimum service reduction and 26 pounds per square inch for full service reduction. The control valve on each rail car senses the reduction in brake pipe pressure and repositions to supply a proportional air pressure from the auxiliary reservoir to a brake cylinder to actuate brake shoes on the rail car. To release the brakes, the operator charges air from the main reservoir to the brake pipe to increase brake pipe pressure. When the control valve on each rail car senses an increase in brake pipe pressure, the control valve repositions to vent brake cylinder pressure through a retaining valve to atmosphere to release the brake shoes. The control valve also repositions to allow brake pipe pressure to re-charge the auxiliary reservoir.
For emergency braking, the operator rapidly vents brake pipe pressure. When any control valve senses a sufficiently rapid reduction in brake pipe pressure, the control valve repositions to supply air pressure from the auxiliary and emergency reservoirs to the brake cylinder to actuate the brake shoes. In addition, the control valve repositions to vent brake pipe pressure locally to more rapidly propagate emergency braking to other rail cars in the train.
The retaining valve on each rail car selectively controls the release of brake cylinder pressure to atmosphere. The retaining valve allows the brake system to maintain a limited braking force when descending long grades while also allowing the brake pipe and associated reservoirs to be recharged for a subsequent brake application. The standard retaining valve is a manually operated, three-position valve, with the functionality of each position prescribed by the AAR Manual of Standards and Recommended Practices Brakes and Brake Equipment, Standard S-446. One position is the Direct Exhaust (EX) position in which the retaining valve rapidly vents brake cylinder pressure to atmosphere, providing a pressure blowdown from 70 psi to 10 psi in not more than 3 seconds. Another position is the Slow Direct Exhaust (SD) position in which the retaining valve slowly vents brake cylinder pressure to atmosphere, providing a pressure blowdown from 60 psi to 40 psi in 17 to 22 seconds. The third position is the HP Retain (HP) position in which the retaining valve slowly vents brake cylinder pressure to atmosphere until the brake cylinder pressure reaches 16-22 psi.
Continuous exposure and operation in the harsh environments associated with railroads accelerates wear of the retaining valve, requiring frequent inspections to repair or replace worn components. The number of internal components and precise fitting between these internal components further complicates the inspection and repairs. As a result, the need exists for an improved retaining valve that is easier to inspect and repair.